The absorption rates of calcium (Ca) and inorganic phosphate (P) will be determined in human intestines by intestinal perfusion. The effect of selected experimental factors, such as the Ca or P concentration, the luminal pH, etc. on these absorption rates will be characterized. the effect of calcitonin (CT), parathyroid hormone (PTH) and the active metabolites of vitamin D on the absorption rates will be studied in normal volunteers and selected patients. The effects on Ca and P absorption produced by exogenous hormone will be compared to the effects associated with the endogenous secretion of the hormone. Patients deficient in one of the 3 hormones, e.g., hypoparathyroidism, and those with a pathologic excess of a hormone, such as medullary thyroid carcinoma or hyperparathyroidism will undergo perfusions to determine the effects on Ca and P absorption of these pathologic states. Patients reported to have abnormalities of Ca and P absorption, such as those with renal insufficiency or vitamin D resistant rickets, will also be studied. The concomitant measurement of cyclic nucleotides in the intestinal lumen and of the absorption rates of water and electrolytes will characterize the mechanism of the intestinal effect produced by each hormone. The addition of radio-isotopes in selected studies will permit the determination of bidirectional fluxes before and during hormone administration. The proposed research will clarify and expand our knowledge of the hormonal control of Ca and P absorption in humans.